1. Field of the Invention
The invention relates to a novel method and apparatus for monitoring the control rod drop times of the control rods of a nuclear reactor.
2. Description of Related Art
Nuclear reactors contain a control rod drive system wherein the core reactivity is maintained by the use of movable control rods interspersed throughout the core. These control rods control the overall reactor power level and provide the principal means of quickly and safely shutting down the reactor. The rods are vertically moved, usually by hydraulic or electromechanical mechanisms. The vertical movement of the rods may be upward into the core, or downward into the core depending on the type of reactor. The drive mechanisms provide both positioning and latching functions, and a scram function with the latter overriding any other signal to promptly shutdown the reactor. The reactor is shut down by fully inserting the control rods in the core. The period required for the control rods to reach the fully inserted position or some intermediate position from the initiation of a reactor trip signal (i.e., scram signal) is referred to as the rod drop time (irrespective of the type of reactor).
Within the United States, the Nuclear Regulatory Commission (NRC) requires in its licensing agreements with power companies operating nuclear reactors that the rod drop time be periodically tested and that the rod drop times be less than some minimum specified time. The rod drop time is typically measured between the initiation of the trip signal and when the rods reach the full insert position and/or when the control rods reach the snubber, which acts as a damper on the movement of the control rod to prevent the rod from impacting the end of its travel path at a speed that might cause damage, or some combination of these times. These tests are usually conducted after the reactor core has been substantially dismantled for periodic maintenance and refueling, to assure that the reactor has been properly reassembled. In the U.S. these tests are conducted in such a manner as to require that special equipment be installed at the time of the test in order to monitor the rod drop times of one or a few of the control rods. The test equipment is then reinstalled to test the drop times of additional control rods until all of the control rods are tested. Naturally, this procedure takes a great deal of time. Due in part to the test interconnections and to its bulk and expense, the test equipment is unsuitable for permanent installation or for measuring the rod drop time of all of the control rods.
Some of the problems with such a testing system are that the rod drop times are not continuously monitored since the test equipment is not permanently installed. Even when the test equipment is installed, not all of the control rods are monitored for rod drop times at one time. Further, the setting up of the test equipment is a relatively time consuming and expensive operation. Further, the time for test may be inconvenient to the operation of the plant.
Another control rod monitoring system is disclosed in German Patent No. 0 214 020. The system includes a device for measuring the control rod drop times of reactor control rods by monitoring the output of acoustic detectors located adjacent the travel path of the control rods. As the rod begins passing an acoustic detector, the detector output exceeds a threshold value and a timer is initiated. Once the rod completely passes the detector, the detector output signal falls below a threshold value and the timer is stopped. In this way, the control rod drop time and the travel time (i.e., speed) of the control rod are measured and displayed. One disadvantage with such a system is that though the detector may be positioned so as to indicate the full rod insert position, it cannot actually register that the control rod is in the full insert position.
Yet another control rod monitoring system is disclosed in Japanese Patent Publication No. 59-0154391. The system includes a high-speed scram control rod measuring apparatus having two sets of reed switches and one additional reed switch controlling a timer, to measure both rod drop time and travel time. The Japanese document discusses as background another rod drop timer wherein a reed switch sends a signal to a buffer time measuring meter. This reed switch registers the full insert position of a control rod. A dedicated timer and a separate display for the buffer time meter are disclosed. However, the signal produced by the reed switch must be processed through the circuitry of a larger system (which also measures the relative speed of the control rods), thereby requiring complicated signal processing circuitry. As the Japanese document points out, this other system requires a substantial amount of hardware which leads to complex and expensive problems.
The buffer time measuring meter of the Japanese invention is apparently initiated by the activation of the reed switch positioned near the insertion position of the monitored control rod (i.e., at the beginning of the control rod's travel path), rather than initiated by the occurrence of a trip signal. Therefore, the rod drop time is not accurately measured with reference to the reactor trip signal and may not be an accurate indication of the full rod drop time. The Japanese devices require a complicated signal processing section and a multiplicity of switches, some of which are cascaded in parallel, to detect the beginning of the control rod insertion and full insertion of the control rod.
In the past, the nuclear industry was faced with a choice of accepting a rod drop time monitoring system which involved complicated signal processing circuitry or otherwise raised questions of reliability, or no system at all. In the United States, a monitoring system has been utilized that had to be periodically installed in the reactor system to test the rod drop time of a selected few control rods at a time, which is expensive and time consuming.
Accordingly, it is an object of the present invention to provide an improved method and apparatus for control rod drop monitoring.
It is a further object of the present invention to provide a control rod drop monitoring method and apparatus that is reliable, efficient and relatively inexpensive to maintain and install.
Still a further object of the present invention is to provide a rod drop monitoring method and apparatus that can be easily and accurately tested.
Yet a further object of the invention is to provide a rod drop monitoring method and apparatus that includes sufficient safeguards against unintentional or unauthorized reset or disruption to the measured quantities.
The present invention achieves these objects and others by providing a control rod drop monitoring method and apparatus which may be permanently installed. The monitoring system includes dedicated timers and displays to measure and display the drop time for each and every control rod in a nuclear reactor anytime a system trip occurs.
The timers are initiated upon receipt of one or a combination of reactor trip signals and are halted at the closure of a limit switch located at the fully inserted or other preselected position of the control rod. The monitoring system is designed for continuous operation so as to capture all the rod drop times of all the control rods in the reactor system.
The reactor trip signal and limit switch signal are fed to a latch circuit following a trip event and are thereby unaffected by subsequent changes in the state of the limit switch or the reactor trip signals.
A further aspect of the present invention is that a manual reset is required via a key operated switch to clear rod drop times thereby providing a measure of security. The key operated switch is also required to enter the test mode thereby offering additional security against tampering or unintentional disruption of the measured quantities.
Another aspect is that the rod drop monitor system is 100% testable in the test mode wherein start and stop push buttons simultaneously activate and simultaneously halt all timers, respectively. Substantially all the circuitry of the monitor system is tested and the accuracy of an internal clock can be verified.
The timers and displays may be of relatively inexpensive digital type produced through CMOS technology and may have millisecond resolution, if desired. The use of CMOS and liquid crystal technologies enables the system to operate at a very low power consumption. Since the monitor system requires no computer hardware or software, involves a relatively simple circuit and can be custom built according to the specifications of a particular reactor, a substantially more reliable system can be offered at a relatively low cost as compared to the large complex systems mentioned above.
The rod drop monitor system of the present invention is accurate while facilitating easy testing of the rod drop time. The monitor system may operate continuously to quickly monitor all rod drop events in real time while fulfilling the essential requirements imposed by the NRC for such tests. The system is reliable and relatively inexpensive to acquire and operate as opposed to the previously mentioned systems. Furthermore, the system provides a measure of safety by displaying the rod drop times, and therefore any irregularity, in a manner which serves to quickly appraise the attendant of the existence of a problem and where in the rod drop system the problem exists. Additionally, the system provides security to assure that the tests are not deliberately or inadvertently altered before they can be officially noted.